1. Field
The invention relates generally to antenna systems, and more particularly to a resonant feed device for coupling a coaxial or parallel wire system in a ground independent and/or feedline independent manner.
2. Prior Art
There are various feed devices for antenna systems, and many integral combinations of feed devices and antenna configurations, available at present which have been designed to improve the radiation characteristics and thus the efficiency of antennas, to physically shorten antennas while retaining their electrical characteristics, to simplify the antenna configurations, to provide improved broadband characteristics, etc. The feed devices employed have proved to be an inherent weak link in the overall system, in that any unbalance in the radiators of the antenna or in the feed lines thereto, cause a corresponding self-destructive increasing cycle of feed line unbalancing, with attendant further increase in the radiator unbalance, etc.
To illustrate, it is commonly known in the art that vertical antennas are generally preferable to horizontal antennas since the former have a much lower radiation angle with respect to the earth, wherein, for example, a transmitting antenna provides a greater range of communication. In addition, a vertical antenna configuration lends itself to mobile installations on moving vehicles as well as a more pleasing appearance in stationary installations in populated areas. However, horizontal antenna configurations have an inherent advantage in that their feed lines drop perpendicularly to the plane of the antenna and thus there is minor interaction between any unbalanced currents in the feed line and the horizontal radiating elements of the antenna.
On the other hand, the feed line coupled to a vertical antenna configuration extends therefrom parallel to the elements thereof, whereby any unbalance currents flowing in the feed line generate a radiation interaction with the antenna, which interferes with the transmission directivity and front-to-back ratio of the radiation pattern. This is particularly true in center fed vertical antennas of any of the various configurations; e.g., dipole, yagi, log periodic, etc. In end fed vertical antennas, the interaction, while not as pronounced, still causes interference with the radiation pattern to greatly reduce the efficiency, i.e., the range of communication of the antenna.
Various schemes have been developed to overcome the difficulties caused by unbalanced radiator elements and/or unbalanced current flows in a feed line, some of which are concerned with the antenna structure itself, and others with the feed line arrangement, configurations, etc.
Typical of antenna structures designed to circumvent the effects of unbalanced currents in the radiating elements are those known as folded dipoles, sleeved or shielded dipoles, etc. Examples of prior art antenna structures which inter alia show the folded sleeved dipole construction are found in U.S. Pat. No. 3,438,042 to J. A. Keucken, U.S. Pat. No. 3,293,646 to H. Brueckmann, and the publication, "Antennas and Transmission Lines," J. A. Keucken, Howard W. Sams & Co., Inc., 1969, cf. Chapter 31, et al.
Other antenna structures employ the integral addition of specific components to electrically alter the antenna, or to provide a feed device, to increase or otherwise match the impedance of the feed line coupled thereto. Such solutions take advantage of the plane of symmetry that exists about the center of a dipole antenna, by making that plane of symmetry ground, and feeding the antenna between the ground and the quarter wave vertical radiator. This supposedly is mathematically the same as having an isolated antenna in free space. The problem is that it is very difficult to obtain an electrically true, absolute ground. Any resistance that occurs becomes a direct loss in the antenna. Thus, impedance matching feed devices which are based on the presumption that the impedance is "high" at the end of the (dipole) antenna, and that the feed line should be transformed from a low impedance to a high impedance to allow efficient feeding of the antenna from its end, are inadequate. The impedance of an antenna at the ends where the antenna current is zero with respect to ground is not definable, since the curl of magnetic and electrostatic fields is not zero. Ergo, the current flow in an antenna can be defined, but the voltage at the antenna end cannot. Therefore, attempts to match the feed line impedance with the "high" impedance of the antenna via diameter relationships, inductors or choke coils, shieldings, etc., are in fact futile, because the addition of the high impedance calculated as that of the antenna is insufficient; the antenna is not fed with respect to ground. It is undesirable to have a current flowing between the end of the antenna and ground. Rather, the antenna should be isolated with respect to ground, i.e., made ground independent. This can only be accomplished by providing a feed device not of matching "high" impedance, but one of overall impedance approaching infinity. It follows that the feed device or devices must be resonant to present an infinite impedance.
Typical of feed devices integral with antenna structures are those shown in the prior art citations of previous mention, as well as in U.S. Pat. Nos. 2,913,722 and 3,100,893 to H. Brueckmann, 2,210,066 to E. C. Cork et al, 2,311,472 to H. O. Roosenstein, and in the United Kingdom Patent Specification No. 690,113 to R. B. Coulson.
Additionally, there are prior art antenna feed devices that employ some form of resonant means, but they all have serious deficiencies that prevent their utilization in practical, efficient antenna structures. Typical of these devices are those depicted in U.S. Pat. Nos. 2,485,457 to R. K. Potter, 3,879,735 to D. V. Campbell, et al, and 2,297,513 to H. J. von Baeyer. These patents, in common, suffer from their incompleteness in presenting a fully operable and practical antenna system. All three use a form of resonant feed device, but none employ a ground connection to their resonant device on the coupling thereto opposite their connection to the antenna. Under these conditions, the feed device, whether resonant or not, becomes only a phase shifting device which couples the rest of the feed line (of undetermined length) to the radiating antenna, drastically changing the resonant frequency and the radiation pattern of the antenna. In addition these prior art resonant feed devices are physically configured to provide resonance using distributed capacitance and/or inductance. Because of this means of establishing resonance it is impossible to accurately determine the location of the current node in the resonant feed device. As a result, the feed device is attached to the antenna at some point other than the current, i.e., electrical, node point, where there is current flow, and the device then interacts with the driven antenna, much like a capacitance hat, thus changing its resonant frequency and its radiation characteristics.
In a further U.S. Pat. No. 2,158,875 to L. M. Leeds, there was suggested the physical arrangement of components for a form of resonant feed device. The latter patent shows that the currents flowing inside a coaxial cable can be very different than those flowing on the outside of the coaxial cable, and that the coaxial cable can be coiled into an inductor, which when resonanted by a shunt capacitor, provides a high impedance between the connection point of the device and the antenna, and ground. However in such device, the frequency of the currents flowing inside of the coaxial cable are not at all related to the frequency of the currents flowing in the antenna structure and in the resonant device, and the device is connected to the (wrong) antenna (i.e., a tower antenna) at a point of well defined impedance, i.e., it is connected in shunt across the output terminals of the transmitter which obviously does not provide infinite impedance.
Typical of another prior art feed device is the balanced-to-unbalanced device, commonly known in the art by the abbreviation "balun," utilized primarily to couple a balanced load to an unbalanced source, i.e., to provide impedance matching over a large range of frequencies. The balun is essentially a transformer whose secondary (to the antenna) is center tapped to ground, and whose primary (to the receiver or transmitter) is connected to a coaxial cable. The shield of the coax is grounded. The balun transformer makes a geometric balanced-to-unbalanced transform, with symmetry about the ground which is common to the primary and secondary windings. The transformer works well when the ground connection is a true RF ground capable of absorbing any unbalanced or circulating currents. It is physically seldom possible, at frequencies less than 50 MHz, to achieve a true RF ground at the feed point of an antenna. Therefore, any unbalanced current in the supposedly balanced load will be directly reflected in the primary current and will result in current flow in the coax shield. Since, in reality, the dipole antenna is seldom perfectly balanced due to asymmetry in the world surrounding the antenna, e.g., trees, buildings, etc., the resulting initial unbalance will cause an increased unbalance due to the interaction of the radiated feed line or coaxial shield currents and the radiated antenna currents. Thus, the balun transformer only works when the secondary load is truly balanced, is non-radiating, or in the special case where the coaxial shield feeding the balun represents a true absolute ground. However, as mentioned above, true absolute grounds are almost impossible to obtain practically, particularly for mobile installations. Furthermore, if a true absolute ground is available at the coaxial shield connection to the antenna, then there is no need for the balun since no interfering radiation is generated in the first place.